There are real differences in the ways men and women think and behave – but which ones matter?

In November last year, Donald Pfaff, a professor of neurobiology from Rockefeller University in New York, was a keynote speaker at a conference titled The Difference Between the Sexes, held at the European Molecular Biology Laboratory in Heidelberg, Germany. This was the latest in the annual Science and Society conference series, which aims to bring topical, cutting-edge biology to public attention - and it clearly hit the mark. This is a hot area of research but Pfaff was not alone in expressing frustration at how the findings are sometimes portrayed. Several other speakers noted that while we are happy to accept that men and women differ physically, when it comes to behaviour or the way we think, sexual politics quickly muddy the picture.

The conference also revealed that when you strip away the politics it is surprising how much we actually do know. There is certainly enough science to address such contentious issues as whether women are innately bad at mathematics (they are not) and whether cultural indoctrination alone can explain why boys and girls tend to play with different toys (it cannot). Yet confused onlookers are often left with the impression that, when it comes to sex differences, everything is still up for grabs. So what's the inside story?

For a start, we have learned a great deal about the biology that underpins sex differences. For years, the accepted view was that all embryos start out the same - the default sex being female. Then during the first trimester, in individuals that have inherited a Y chromosome, a gene called sry, for sex-determining region Y, switches on the development of the testes. These start pumping out testosterone and by the time a baby boy is born, the "default" female brain has become masculine.

We now know that's not quite how it works. It turns out there are "pro-female" as well as "pro-male" genes, and that sexual differentiation is governed by a delicate balance between the two. In 2006, for example, Pietro Parma at the University of Pavia in Italy, and colleagues, reported that a gene called r-spondin1 promotes the development of the ovaries, and that without it individuals who are genetically female grow up physically and psychologically male, although they have ambiguous external genitalia and are sterile (Nature Genetics, vol 38, p 1304).

Biologists have also revised their views on the role of sex hormones. Testosterone in men and oestrogen in women were always thought to account for most of the biological differences between the sexes. While that remains the mainstream view, it is now clear that the effects of hormones and genes can interact, with implications for the wiring of the brain and, ultimately, for behaviour. Moreover, the contribution of genes can in turn be modified by experience: a child's early environment can induce chemical modifications of DNA - so-called epigenetic changes - that without altering the actual sequence of a gene changes whether it is active or quiescent in a particular tissue.

The identification of all these sex-determining factors and their complex interactions has an important corollary, which is that sex determination is not over by birth, as was once thought. Both nature and nurture play a role in shaping the differences between men and women, nowhere more so than in the brain, which is constantly remoulded throughout our lives. Many now believe that there are critical periods when the sex of a child's brain - and everything that accompanies it, including such things as the individual's attitudes to love or food - is particularly malleable. By the time we reach adulthood there are numerous differences in structure between the brains of men and women, as revealed by brain-imaging studies (see diagram). These could explain why males and females show such different vulnerabilities to mental illness and learning difficulties (see "Wired for trouble"), but as yet neuroscientists know little about how the structural differences translate into behaviour.

Spot the difference

That said, over the years psychologists have developed a good picture of which human behaviours show sex differences. What has emerged is a hierarchy of traits within which, Pfaff noted at the Heidelberg conference, there is one rather obvious pattern: "The further you go from reproductive behaviour, the less impressive the sex differences." So, not surprisingly, at the top of the table are gender identity and sexual orientation, which both have a direct bearing on an individual's chances of reproducing. Put simply, the vast majority of people who think of themselves as male are men, while those who consider themselves female overwhelmingly tend to be women. Likewise, most people who prefer their sexual partners to be women are men (and vice versa).

Nobody is going to object to that. But things get more contentious further down the scale when we start considering traits such as empathy and assertiveness. One way to cut through this is by comparing the extent of psychological differences between men and women with an obvious physical one such as height. As well as putting the size of various behavioural differences into perspective, this also gives a more dispassionate take on what the differences mean. Everyone would agree, based on their visual experience, that men are on average taller than women, yet there are enough tall women and short men in the world that height alone is not a reliable predictor of an individual's sex. A similar rationale exists for behavioural differences.

Taking this approach, last year Jay Giedd and Judith Rapoport of the US National Institute of Mental Health in Bethesda, Maryland, pointed out that most of the effects of sex on behaviour are only around half the size of those on height (Neuron, vol 67, p 728). Nevertheless, there are real and measurable differences. Play behaviour, for example, while varying far less between the sexes than gender identity or sexual orientation, is next in the league and differs by about the same amount as height. In practice this means that boys are on average more likely than girls to engage in rough-and-tumble play, or to choose a truck over a doll, but there are enough exceptions to that rule that it is not possible to predict a child's sex from his or her play preferences alone (see "Toy story").

The areas where differences between men and women are about half that of height include aggression, empathy, assertiveness and cognitive skills such as the ability to mentally rotate an object. Further down the list come verbal fluency and mathematical attainment, which show far less variation between the sexes than we are often led to believe. And at the bottom of the chart are a bunch of traits commonly thought to be biased by sex but which in practice show no discernible difference between men and women. These include computational skills, overall verbal ability and leadership potential.

In other words, the picture science paints is one where sex differences are real but not deterministic. In certain areas men may tend to be one way and women another, but the role played by nurture and the environment in shaping these differences means that we may have more influence over them than we thought. That is certainly good news for anyone who believes that the workplace should be more conducive to women, and the classroom more boy-friendly. But there are no simple fixes, warns child psychologist and writer Susan Pinker, based in Montreal, Canada, who also spoke at the Heidelberg conference. There may nevertheless be things we can do - and a readiness to leave the politics aside and address the issues empirically is a good starting point. It is no longer good enough to say that boys will be boys, and girls will be girls. "That," says Pinker, "is simply the path of least resistance."